The average cost of a gallon of gas in January 2014 was $3.42, and was $2.36 in December 2014. What was the percent change in the average cost of a gallon of gas in 2014? Round to the nearest percent. The average cost of a gallon of gas in January 2014 was $3.42, and was $2.36 in December 2014. What was the percent change in the average cost of a gallon of gas in 2014? Round to the nearest percent.